The cause of ME is unknown, although the evidence suggests that it is either a persistent viral infection or a dysfunction of the immune system.
It is associated by many people with adult high-flyers - in the 1980s the tabloid newspapers dubbed it "yuppie flu" - but its prevalence among children has caused increasing concern. Action for ME says it may affect as many as 24,000 children.
The best known - and usually the worst - symptom is extreme weakness, but it also slows the brain and interferes with memory. Other symptoms include muscle pain, sleep disturbance and mood swings. This is how Action for ME describes the condition: "Imagine you had a severe hangover, plus daily influenza, muscle pain and exhaustion as though you had run a marathon, and could not think or comprehend anything for weeks, months or even years . . . this is what having ME is like."
Children with ME are often wrongly diagnosed as depressed, malin-gering or school phobic. Instead of being taught the best way to manage their condition by resting and pacing themselves, many are told to pull their socks up and exercise - which makes the illness worse. Rest is the only known treatment.
The average duration of ME among children is four or five years.